import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/sysmon/loop_unittest.py b/scripts/sysmon/loop_unittest.py
index 5c7bb2a..6e53cf0 100644
--- a/scripts/sysmon/loop_unittest.py
+++ b/scripts/sysmon/loop_unittest.py
@@ -13,10 +13,9 @@
 import contextlib
 import sys
 
-import mock
-
 from chromite.lib import cros_test_lib
 from chromite.scripts.sysmon import loop
+from chromite.third_party import mock
 
 
 assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'